Feast-famine enrichment of mixed microbial cultures enhances wastewater valorization into bioplastics

Polyhydroxyalkanoates (PHAs) are biodegradable biobased polymers with the potential to replace conventional plastics. To reduce production costs while contributing to wastewater treatment, mixed microbial cultures (MMCs) have been proposed as a sustainable platform for PHA production. This process comprises three stages: (1) acidogenic fermentation of wastewater streams to generate volatile fatty acids (VFAs), (2) selection and enrichment of MMCs under feast-famine conditions, and (3) PHA accumulation using the selected biomass. The aim of this study was to evaluate the selection and enrichment of MMCs for PHA production within a circular economy framework. VFAs obtained by sludge acidogenic fermentation were used as the carbon source, and two operational factors organic load (500-1500 mg COD L^-1) and cycle length (3 or 6 days) were tested in a $3\times 2$ factorial design. The condition of 500 mg COD L^-1 with a 3-day cycle achieved the highest performance, with statistically significant effects (p < 0.05) for both main factors and their interaction. Pilot-scale validation in a 16-L working volume reactor confirmed these results, yielding 83.3 mg PHA L^-1 (reported as PHB-equivalents following acid hydrolysis to crotonic acid and HPLC quantification) and a yield of 0.28 g COD g COD^-1 ( $Y_{PHA/VFA}$ , expressed as COD-based PHA produced per COD of VFA consumed). Molecular analysis revealed species classically associated with PHA dynamics (Paracoccus sp., Alcaligenes sp.) and the presence of Diaphorobacter limosus, suggesting complementary roles under selective pressure. These findings highlight wastewater valorization into bioplastics as a viable route within the circular economy framework.